Recently, researchers form Chinese Academy of Agricultural Sciences (CAAS) and South China Agricultural University (SCAU) reported a method named Haploid-Inducer Mediated Genome Editing (IMGE) for accurately accelerating crop breeding. The method and results were published on the leading international journal “Molecular Plant” titled as “Development of a Haploid-Inducer Mediated Genome Editing (IMGE) System for Accelerating Maize Breeding” on March 20, 2019.

Crop breeding aims to generate pure inbred lines with multiple desired traits. Doubled haploid (DH) and genome editing using CRISPR/Cas9 are two powerful game-changing technologies in crop breeding. However, both of them still fall short in rapid generation of pure elite inbred lines with integrated favorable traits. Recently, researchers developed a Haploid-Inducer Mediated Genome Editing approach (IMGE for short), which utilizes a maize haploid inducer (HI) line carrying a CRISPR/Cas9 cassette targeting for a desired agronomic trait to pollinate an elite maize inbred line, and to generate genome edited haploids in the elite maize background. As the chromosomes from HI, which carrying the CRISPR/Cas9 cassette, are excluded in the process of haploid induction, the yielded haploid is transgene-free. Homozygous pure DH lines with the desired trait improvement could be generated within two generations, thus bypassing the lengthy procedure of repeated crossing and backcrossing used in conventional breeding for integrating a desirable trait into elite commercial backgrounds.

Given that most crops are recalcitrant to transformation and the efficiency is often genotype-dependent, the authors envisage that the IMGE system should be widely applicable to numerous crops. This technology allows for direct editing of the genomic targets in any elite crop background, thus bypassing the laborious and lengthy introgression process. In addition, the produced genetic materials are transgene-free, and thus should not involve transgenic oversight and minimize the requirement of regulatory approval. A further improvement of this system could be achieved by integrating the recently reported oil content and double-fluorescence protein (eGFP and DsRED) marker to facilitate the identification of maternal haploid seeds with much higher efficiency and accuracy. Meanwhile, as multiple gRNAs can be stacked into a single CRISPR/Cas9 cassette, this technology should allow editing of multiple genome sites simultaneously to pyramid multiple favorable alleles in a single elite background. Further, the development of new genome editing technology (base editing, gene replacement and knock-in etc.) should greatly expand the utility of this strategy. Thus this method, just like a magician's wand, may accurately accelerate crop breeding.

As the potential application prospect of this method, the editorial office of “Molecular Plant” published a report named “Next-Generation Crop Breeding Methods”, highlight this invention. IMGE, together with other two recently reported approaches, likely constitute a part of next-generation crop breeding methods. With the development of these highly promising approaches and further improvements, we trust a new era of crop breeding is coming of age.

By: Wang Baobao 

wangbaobao@caas.cn